MODELING INFILTRATION INTO THE UNSATURATED VASHON ADVANCE OUTWASH FROM CLASS-V UIC WELLS

Class-V underground injection control (UIC) wells are a practical and cost-effective technique to achieve Washington State Department of Ecology stormwater management goals to maximize infiltration for either new site development or redevelopment. UIC wells can be used to convey treated stormwater runoff into the subsurface through relatively impermeable deposits at the surface to an adequate infiltration receptor underneath. In the Puget Lowlands, UIC wells typically infiltrate into the unsaturated portion of Vashon Stade advance outwash (Q_va) of the Fraser Glaciation. The Q_va is a glacio-fluvial deposit composed of proglacial and glacially-derived sand and gravel, generally coarsening upwards, with localized zones of finer sand and silt. The presence of low hydraulic conductivity beds throughout the Q_va decreases vertical hydraulic conductivity and can be detrimental to the potential infiltration capacity of these wells. Modeling provides an opportunity to address the effects of Q_va anisotropy and unsaturated thickness on the extent of local groundwater mounding and horizontal flow paths. SEEP/W, by GEO-SLOPE International Ltd., is a finite-element CAD software which can be used to simulate subsurface conditions during borehole infiltration tests of UIC wells using a transient unsaturated-saturated model with user defined boundary conditions. Future infiltration studies could use this information as part of well interference and down gradient impact assessment of UIC wells to evaluate the adequacy of proposed infiltration facility design and the potential impacts to slope stability located within the zone of influence of the infiltration system.